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    SubjectRe: [PATCH v2 4/5] mm: introduce MADV_PAGEOUT
    On Thu 20-06-19 17:40:40, Minchan Kim wrote:
    > > > > Pushing out a shared page cache
    > > > > is possible even now but this interface gives a much easier tool to
    > > > > evict shared state and perform all sorts of timing attacks. Unless I am
    > > > > missing something we should be doing something similar to mincore and
    > > > > ignore shared pages without a writeable access or at least document why
    > > > > we do not care.
    > > >
    > > > I'm not sure IIUC side channel attach. As you mentioned, without this syscall,
    > > > 1. they already can do that simply by memory hogging
    > >
    > > This is way much more harder for practical attacks because the reclaim
    > > logic is not fully under the attackers control. Having a direct tool to
    > > reclaim memory directly then just opens doors to measure the other
    > > consumers of that memory and all sorts of side channel.
    >
    > Not sure it's much more harder. It's really easy on my experience.
    > Just creating new memory hogger and consume memory step by step until
    > you newly allocated pages will be reclaimed.

    You can contain an untrusted application into a memcg and it will only
    reclaim its own working set.

    > > > 2. If we need fix MADV_PAGEOUT, that means we need to fix MADV_DONTNEED, too?
    > >
    > > nope because MADV_DONTNEED doesn't unmap from other processes.
    >
    > Hmm, I don't understand. MADV_PAGEOUT doesn't unmap from other
    > processes, either.

    Either I am confused or missing something. shrink_page_list does
    try_to_unmap and that unmaps from all processes, right?

    > Could you elborate it a bit more what's your concern?

    If you manage to unmap from a remote process then you can measure delays
    implied from the refault and that information can be used to infer what
    the remote application is doing.
    --
    Michal Hocko
    SUSE Labs

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